Inductor for integrated circuit

ABSTRACT

An inductive component ( 1 ), especially intended to be incorporated into a radiofrequency circuit, comprising:  
     a substrate layer ( 2 );  
     a flat inductor formed from a metal strip ( 3 ) wound in a spiral;  
     wherein:  
     the substrate layer ( 2 ) is made of quartz;  
     the metal strip ( 3 ) is made of copper and has a thickness (E 3 ) of greater than 10 microns.

TECHNICAL FIELD

[0001] The invention relates to the field of microelectronics andmicrosystems. It relates more specifically to a novel design ofinductive components or integrated transformers which are intended to beassociated with integrated circuits, such as those used especially inthe radiofrequency field.

[0002] The purpose of the invention is more specifically to obtaininductors having a better Q-factor right from low frequencies andoperating at higher frequencies than with inductors currently obtained.

PRIOR ART

[0003] As is known, integrated circuits are being used increasingly inthe microwave and radiofrequency fields.

[0004] In these applications, it is important to be able to use tunedoscillating circuits consisting of a capacitor-inductor combination.However, these circuits must be produced so as to occupy increasinglysmaller volumes. Furthermore, they must operate at increasingly higherfrequencies.

[0005] Finally, the electrical consumption of such components isbecoming a critical parameter, for example in cellular portabletelephones, since this consumption has a direct influence on theautonomy of these appliances.

[0006] Thus, the passive components forming the filters used inradiofrequency systems, especially the inductors, are required to occupyas small as possible an area within the integrated circuits, with thisinductor having as high an inductance as possible, and to result in aslow as possible an electrical consumption.

[0007] Furthermore, it is known that the inductors fitted intointegrated circuits made of semiconductor material are exposed to theinfluence of parasitic capacitances formed by the various localizedsubstrate regions in the vicinity of the inductors.

[0008]FIG. 1 shows an equivalent circuit for an inductor implanted in anintegrated-circuit chip.

[0009] Thus, such an inductor has an equivalent circuit in which, addedto the actual inductor (L), there are the various parasitic componentswhich make this inductor depart from its ideal behavior.

[0010] Thus, an inductor has a resistance (R_(s)) corresponding to thatof the metal of which it is composed. Furthermore, this inductor hasvarious capacitances (represented by the capacitors C_(i) and C_(O))which are parasitic capacitances resulting from the presence of theoxide layer deposited on top of the substrate. Furthermore, thisinductor includes, in series with said capacitors C_(O) and C_(i), acapacitor C_(sub) and a resistor R_(sub) in parallel, corresponding tothe influence of the semiconductor substrate located between said oxidelayer and the ground plane.

[0011] Moreover, a certain parasitic capacitance exists between thevarious turns making up the inductor and is modeled in the equivalentcircuit in FIG. 1 by the capacitor (C_(s))

[0012] The Applicant, in French Patent Application FR 98/08434, which atthe date of filing of the present application has not yet beenpublished, has described a solution allowing such inductors to beproduced on a semiconductor substrate by adopting an arrangementallowing the capacitance of the interturn parasitic capacitor (C_(s)) tobe greatly reduced. By virtue of such a solution it is possible to usesuch an inductor at higher frequencies, while still retainingsatisfactory behavior.

[0013] It will be recalled that the optimum operating frequency isdefined as being that for which the Q-factor is a maximum. The Q-factoris defined in a known manner by the ratio of the imaginary part orreactants to the real part of the input inductance corresponding to themodel illustrated in FIG. 1.

[0014] The solution described in the aforementioned document, althoughsatisfactory, does not allow the Q-factor to be significantly improvedin the low-frequency ranges, that is to say those lying below half theoptimum frequency, which is typically close to a few gigahertz in theradiofrequency applications of the invention.

[0015] This is because, in this frequency range, the behavior of theinductor is strongly dependent on the value (R_(s)) of the resistor,which corresponds to the electrical resistance of the metal strip fromwhich the actual inductor is composed.

[0016] However, all the inductors produced in circuits, even integratedcircuits, are made at the present time of aluminum, and are small insize having especially a very small thickness, thereby resulting in ahigh electrical resistance.

[0017] Thus, one of the problems that the invention aims to solve isthat of the undesirable influence of the overall resistance of thewinding forming the inductor.

SUMMARY OF THE INVENTION

[0018] The invention therefore relates to an inductive component,especially:

[0019] a substrate layer;

[0020] a flat inductor formed from a metal strip wound in a spiral.

[0021] The inductive component according to the invention is onewherein, on the one hand, the substrate layer is made of quartz andwherein, on the other hand, the metal strip is made of copper and has athickness of greater than 10 microns.

[0022] In this way, the value of the resistance of the winding is verygreatly reduced by using a conductive material much less resistive thanthe aluminum used in the prior art.

[0023] Furthermore, the chosen dimensional parameters, especially thethickness of the strip of which the turns are composed, also verygreatly reduce the overall value of the resistance of the winding.

[0024] This reduction in the resistance is accomplished while retainingan extremely tiny parasitic capacitance (C_(s)) by virtue of theimplantation of the metal strip on a substrate made of quartz, thedielectric properties of which are quite close to those of air.

[0025] By virtue of the characteristics of the inductor according to theinvention, it has been observed that the Q-factor was ten times greaterthan that observed with an inductor of the same inductance but producedaccording to the prior art.

[0026] By way of example, for an inductance of about 5 nanohenries (nH)and for a typical frequency of 1.8 gigahertz (GHz), the Q-factor isabout 40 whereas with the prior technologies it was close to 4.

[0027] In practice, it has been found that the results are verysatisfactory when the thickness of the metal strip is about 30 microns.

[0028] According to one feature of the invention, the space between theopposing faces of two adjacent turns is free of material.

[0029] Consequently, the presence of air, which has a very lowelectrical permitivity, greatly limits the interturn parasiticcapacitances which have been seen to have a negative impact on theoptimum use of integrated inductors.

[0030] In one embodiment, the inductive component according to theinvention furthermore includes a polyimide layer interposed between theinductor and the quartz substrate, within which polyimide layer thatsegment of the strip which connects the center of the spiral passes, andthe end of the strip forming a connection terminal.

[0031] In practice, the metal strip is advantageously covered with alayer of gold on its faces other than those in contact with thesubstrate or the polyimide layer.

[0032] Thus, the risks of oxidation, inherent in the operation of thecomponent according to the invention in a chemically aggressiveenvironment, such as especially a wet, or indeed maritime, atmosphere,are overcome by protecting the conducting strip from the oxidationphenomena which would degrade the overall resistance of the strip.

[0033] The aforementioned aspects of the invention may also apply to theproduction of integrated transformers. Thus, such a transformer includestwo flat inductors according to the invention, formed by two metalstrips wound in spirals, said spirals being wound in each other so thatthe turns of one of the inductors are positioned between the turns ofthe other inductor.

[0034] In order to obtain high inductances, while still retaining asatisfactory Q-factor, in practice the strip is advantageously wound intwo spirals in series, the two spirals being parallel to each other, thespiral closer to the substrate being embedded in a polyimide layer.

[0035] Consequently, by virtue of the magnetic coupling phenomena, theinductance is more than twice the inductance of an inductor formed froma single spiral. The overall size of such an inductor is thereforereduced. Furthermore, for a given inductance, two spirals are used, eachhaving a resistance less than half of that of a single inductor,something which proves to be advantageous with regard to the Q-factor.

[0036] In one particular embodiment, intended for applications inparticularly aggressive media, the polyimide layer in which the secondspiral is embedded may be covered with a barrier layer made of silica.Consequently, the lifetime of such a component may be extended.

[0037] In one particular embodiment, the inductor according to theinvention is such that the two ends of the strip form connectionterminals on which spacer elements or “bumps” having a height close tothe thickness of the metal strip are mounted.

[0038] Consequently, such an inductor, or its transposition into thetransformer, has elements making it easier for it to be fitted onto anintegrated circuit.

[0039] The invention also relates to an integrated circuit associatedwith an aforementioned inductive component or integrated transformer.

[0040] According to one feature of the invention, such an integratedcircuit comprises connection leads for the inductive component or forthe integrated transformer and it is characterized in that:

[0041] the substrate used is made of quartz;

[0042] the metal strips are made of copper and have a thickness ofgreater than 10 microns.

[0043] In other words, the inductive component or the transformer aremounted using a technique known as “flip-chip”.

[0044] In practice, the spacer elements, usually called “bumps”, areadvantageously chosen so that they have a height close to the thicknessof the strip of the inductive component.

[0045] Consequently, the strip forming the inductor is separated fromthe semiconductor substrate of the integrated circuit by a distancewhich ensures that the inductive component on the integrated circuit ismechanically stable, while limiting the influence of the parasiticconductivity of the semiconductor substrate on the behavior of theinductor.

[0046] This is because it has been observed that if the height of thespacer elements is too great, typically greater than the thickness ofthe strip, this then results in a risk of mechanical instability whichmay lead, in the case of shocks, to the connection between theintegrated circuit and the inductor being broken.

[0047] Conversely, when the distance between the semiconductor substrateof the integrated circuit and the inductor is too small, phenomena ofelectrical loss through the semiconductor are observed, which reduce theperformance of the latter.

[0048] Thus, in practice, good results are obtained with 30 micronspacers or “bumps” for a strip thickness of about 30 microns.

[0049] In practice, in order to further improve the stability of theinductive component mounted on the integrated circuit, the spacerelements are advantageously in the form of a cylinder which preferablyhas a diameter close to three times its height.

BRIEF DESCRIPTION OF THE FIGURES

[0050] The manner in which the invention is realized and the advantageswhich stem therefrom will become clearly apparent from the descriptionof the following embodiments, supported by the appended figures inwhich:

[0051]FIG. 1 is an electrical circuit diagram modeling the behavior ofan inductor mounted on a substrate;

[0052]FIG. 2 is a top view of an inductive component according to theinvention;

[0053]FIG. 3 is a cross-sectional view of a component according to theinvention, using a strip forming a single spiral;

[0054]FIGS. 4 and 5 are partial sectional views illustrating embodimentsin which the inductor is formed by two parallel spirals;

[0055]FIG. 6 is a top view of an integrated transformer according to theinvention, which includes two windings;

[0056]FIG. 7 is a cross-sectional view of a transformer producedaccording to the invention;

[0057]FIG. 8 is a simplified explanatory diagram showing the associationof an inductive component according to the invention on an integratedcircuit;

[0058]FIG. 9 is the reproduction of a photograph, taken in an electronmicroscope, of a metal strip used in an inductive component according tothe invention.

[0059] As already stated, the invention relates firstly to an inductivecomponent intended to be used in association with an integrated circuit,in radiofrequency or microwave applications.

[0060] According to one feature of the invention, this inductivecomponent (1), as illustrated in FIG. 2, essentially comprises asubstrate plate (2), made of quartz, on which a winding consisting of ametal strip (3) is placed.

[0061] This strip (3) is wound in a spiral so as to form an inductivecoil. The shape illustrated in FIG. 2 is a winding in a circular spiral,which is the preferred geometry, but the invention is not limited tothis type of winding and encompasses other spiraled windings in whichthe strip consists, for example, of a succession of straight segmentseach making an angle of 90° with the following segment in order to givethe winding an overall square shape. Other polygonal shapes may also beenvisaged.

[0062] According to an important feature of the invention, the substrateused is made of quartz.

[0063] According to another important feature of the invention, thestrip forming the inductor is made of copper and has a thickness muchgreater than that of the strips used to form the existing integratedinductors.

[0064] Thus, the strip (3) has a thickness (E₃) of greater than 10microns, and preferably close to 30 microns, values to be compared withthe 2 to 3 microns for the thickness of the aluminum strips of existinginductors.

[0065] More specifically, and as illustrated in FIG. 3, the quartzsubstrate layer is covered with a polyimide layer (5) placed between thequartz substrate (2) and the strip (3) forming the inductor.

[0066] In particular, passing through this polyimide layer is thesegment (8) which connects the center (9) of the spiral to the end (10)of the strip, which forms one of the two connection terminals.

[0067] In practice, the thickness (E₂) of the polyimide layer (5) isabout 35 microns. The thickness of the polyimide layer which covers thesegment (8) is between 4 and 6 microns.

[0068] As illustrated in FIG. 3, the width (D2) of each turn (10) isabout 30 microns, while the distance (D1) separating the copper turns(15) is about 30 microns.

[0069] Depending on the value of inductance desired, a spiral having theappropriate number of turns is produced.

[0070] As already stated, according to one of the features of theinvention, the copper strip (3) is covered, on its faces in contact withthe external medium, with a thin gold layer (19), typically having athickness of 1000 to 2000 Å, the function of the gold layer being toprotect the copper strip (3) from oxidation phenomena which, as isknown, degrade the conducting properties of copper.

[0071] According to one feature of the invention, the space (17) lyingbetween the turns (15, 16) of the strip is free of any material, and istherefore filled with air. Thus, the parasitic capacitance existingbetween turns (15, 16) is reduced as far as possible, thereby allowingthe inductor to operate over a greater frequency range.

[0072] If a high inductance is sought, the copper strip is wound so asto form two spirals in series, as illustrated in the cross section inFIG. 4.

[0073] More specifically, the strip forms two flat spirals (21, 22)placed so as to be parallel to each other.

[0074] These two spirals (21, 22) are connected in series at theircommon central axis (23) and rotate in the same direction.

[0075] Thus, as illustrated in FIG. 4, the spiral (22) placed closer tothe quartz substrate (2) is embedded in the polyimide layer (26) whilethe spiral (21) is in contact with the external medium and, like thesingle spiral in FIG. 3, is covered with a gold layer (19) intended toprevent the risk of oxidation.

[0076] The cross section of the turns is identical in both spirals. Inthe particular case in which the spirals are polygonal, it isadvantageous for the turns of the two spirals to be offset transverselyso that the turns are not on top of each other, thereby reducing theinterturn parasitic capacitance.

[0077] In one particular embodiment illustrated in FIG. 5, the polyimidelayer (36) within which the lower spiral (32) is immersed is itselfcovered with a protective silica layer (37) intended to protect thepolyimide layer (36) from external chemical attack and thus to extendthe lifetime of the inductive component.

[0078] The principle of the invention also allows the production ofintegrated transformers which, according to FIG. 6, include at least twoinductors (50, 51) formed by two independent strips and wound inimbricated spirals in such a way that these inductors (50, 51) arecoupled.

[0079] In practice, as illustrated in FIG. 7, the inductors (50, 51) arewound in such a way that the turns (53, 55) of a first spiral (51) areinserted between the turns (52, 54) of the other spiral (50).

[0080] As already stated, the inductive component according to theinvention is intended to be incorporated into a radiofrequency circuit,especially one incorporating integrated circuits.

[0081] Such an association is especially illustrated, schematically, inFIG. 8.

[0082] Thus, the aforementioned inductive component (1) is mounted,using a mounting technique commonly called “flip-chip”, on an integratedcircuit (40), which comprises metal contact pads (41, 42) providedwithin the passivation layer (43) of the semiconductor substrate, bymeans of spacer elements (45, 46) which are generally called “bumps”.

[0083] In FIG. 8, the quartz substrate (2) of the inductive component(1) has been made transparent in order to reveal the strip (3) formingthe inductor, which strip lies between the integrated circuit (40) andthe quartz substrate (2).

[0084] According to one feature of the invention, this mounting iseffected by means of spacers (45, 46) which have a cylindrical crosssection intended especially to ensure good mechanical stability of theinductive component (1) on the integrated circuit (40).

[0085] This is because it is known that radiofrequency circuits have towithstand particularly high mechanical vibration/shock stresses.

[0086] Thus, for example in the application of the invention to portabletelephones, the equipment is subjected to very rigorous impact tests inwhich accelerations of several g may be reached. It is thereforeessential that the components mounted on top of the integrated circuitsare not exposed to any risk of debonding, which would have theconsequence of an appliance being taken out of service. The vibrationstresses are also very severe.

[0087] Thus, according to one feature of the invention, the spacerelements (45, 46) have a particularly robust cylindrical shape. Thediameter chosen for such cylindrical spacer elements is substantiallysimilar to the width of a standard contact pad (41, 42) produced on anintegrated circuit, namely of the order of 100 microns.

[0088] Consequently, this arrangement ensures that there is very goodmechanical stability and the electrical resistance of such a spacerelement (45, 46) is negligible.

[0089] The height of the spacer element is chosen to be very close tothe height of the turns (15) of the copper strip (3) and is typicallyabout 30 microns. Attachment to the integrated circuit takes place bysoldering.

[0090] A height of the spacer element (45, 46) of 30 microns makes itpossible to ensure a good compromise between the mechanical stability ofthe inductive component (1) on the integrated circuit (40), whilelimiting as far as possible the electrical loss phenomena within thesemiconductor substrate that too close a proximity would create.

[0091] The production of the copper strip is carried out using a processwhich incorporates an electrodeposition step. This electrodepositiontakes place by growth on a base line formed on the outer face by a metalgrowth layer covered with a photosensitive resin, which is exposed anddeveloped to the form of the pattern.

[0092] Large turn heights are obtained since the side walls of theheat-sensitive resin form blocks which occupy the volume that willconstitute, after electrolytic growth of the copper, the interturnspace.

[0093] Thus, the electrolytic growth of the copper is channeledlaterally, making it possible to obtain copper turns of almostrectangular cross section and of large height.

[0094] After the step of producing the turns by electrolytic growth, thematerial that had served as a block is removed in order to obtain,according to one feature of the invention, an interturn space filledwith air. The aforementioned metal growth layer is also removed.

[0095]FIG. 9 is a photograph, taken in an electron microscope, whichshows a strip (70) produced according to the aforementioned process. Itis surrounded by two pieces (71, 72), also made of metal, which play norole in the inductive behavior of the component.

[0096] It is apparent from the foregoing that the inductive component,as well as the transformer according to the invention, has manyadvantages, especially an excellent Q-factor which is greater by afactor of about 10 than that of inductors produced according to thetechniques known to date. Furthermore, the operating frequency range ishigh since the interturn parasitic capacitance is particularly low.

[0097] Moreover, the resistance to chemically aggressive atmospheres isvery good.

[0098] When associated with an integrated circuit, by means of thecylindrical spacer elements according to the invention, very goodmechanical stability and resistance to shocks and vibrations areobtained, while still retaining excellent electrical properties,especially in terms of electrical losses within the semiconductorsubstrate of the integrated circuit.

INDUSTRIAL APPLICATIONS

[0099] The inductive components of the transformers according to theinvention may have many applications, and especially in any circuitwhich includes oscillators, amplifiers or mixers, as well as in anyactive or passive filter. The inductive components may also be used bythemselves, as a discrete component, where their high precision can beappreciated.

[0100] The association with integrated circuits is most particularlyapplicable in electronic circuits intended for telecommunication,microwave and radiofrequency processing.

1. An inductive component (1), especially intended to be incorporatedinto a radiofrequency circuit, comprising: a substrate layer (2); a flatinductor formed from a metal strip (3) wound in a spiral; wherein thesubstrate layer (2) is made of quartz; the metal strip (3) is made ofcopper and has a thickness (E₃) of greater than 10 microns.
 2. Thecomponent as claimed in claim 1, wherein the metal strip (3) has athickness (E₃) of about 30 microns.
 3. The component as claimed in claim1, which includes a polyimide layer (5) placed between the inductor (3)and the quartz substrate (2), within which polyimide layer that segment(8) of the strip (3) which connects the center (9) of the spiral passes,and an end (10) of the strip (3) forming a connection terminal.
 4. Thecomponent as claimed in claim 3, wherein the metal strip (3) is covered,on its faces other than those in contact with the substrate (2) or thepolyimide layer (5), with a layer of gold (19).
 5. The component asclaimed in claim 1, wherein the space (17) between the opposing faces oftwo adjacent turns (15, 16) is free of material.
 6. The component asclaimed in claim 1, wherein the strip (3) is in the form of a Circularspiral.
 7. The component as claimed in claim 1, wherein the strip (3) iswound in two spirals (21, 22) in series, the two spirals (21, 22) beingparallel to each other, the spiral (22) closer to the substrate (2)being embedded in a polyimide layer (26).
 8. The component as claimed inclaim 7, wherein the polyimide layer (26) is covered with a barrierlayer made of silica.
 9. The component as claimed in claim 1, whereinthe two ends of the strip form connection terminals on which spacerelements (45, 46) having a height close to the thickness (E₃) of themetal strip (3) are mounted.
 10. An integrated transformer, especiallyintended to be incorporated into a radiofrequency circuit comprising: asubstrate layer (2); two flat inductors (50, 51) formed by two metalstrips wound in a spiral, said spirals being wound one in the other sothat the turns (53, 55) of one of the inductors (51) are positionedbetween the turns (52, 54) of the other inductor (50), wherein: thesubstrate layer (2) is made of quartz; the metal strips are made ofcopper and have a thickness of greater than 10 microns.
 11. Theintegrated transformer as claimed in claim 10, wherein the metal stripshave a thickness of about 30 microns.
 12. The integrated transformer asclaimed in claim 10, which includes a polyimide layer (56) placedbetween the inductors (50, 51) and the quartz substrate (2), withinwhich polyimide layer the segments (58) of the strips connecting thecenters of the inductors (50, 51) pass, and the ends of the stripsforming a connection terminal.
 13. The transformer as claimed in claims10 and 12, wherein the metal strips are covered, on their faces otherthan those in contact with the substrate or the polyimide layer, with alayer of gold.
 14. The transformer as claimed in claim 10, wherein thestrips are in the form of circular spirals.
 15. The transformer asclaimed in claim 10, wherein the space between the opposing faces of theturns forming the strips is free of material.
 16. An integrated circuitassociated with an inductive component as claimed in one of claims 1 to9 or an integrated transformer as claimed in claims 10 to 15, wherein:the integrated circuit (40) includes contact pads (41, 42); and saidinductive component (1) or said integrated transformer is mounted on theintegrated circuit (40) by means of conducting spacer elements (45, 46)ensuring electrical connection between said contact pads (41, 42) of theintegrated circuit and the connection terminals (10) of the inductivecomponent or of the integrated transformer, said inductive component orintegrated transformer being oriented with respect to the integratedcircuit in such a way that its face including the inductors (3) isfacing the integrated circuit (40).
 17. The integrated circuit asclaimed in claim 16, wherein the spacer elements (45, 46) have a heightclose to the thickness of the strip (3) of the inductive component or ofthe integrated transformer.
 18. The integrated circuit as claimed inclaim 16, wherein the spacer elements (45, 46) are in the form of acylinder.
 19. The integrated circuit as claimed in claim 18, wherein thespacer elements (45, 46) have a diameter whose value is close to threetimes that of their height.